The dance of dust: Investigating young stellar object dipper variability^★

¹ University College Dublin (UCD), Department of Physics, Belfield, Dublin 4, Ireland
² Dublin Institute for Advanced Studies, Astronomy & Astrophysics Section, 31 Fitzwilliam Place, Dublin 2, D02 XF86, Ireland
³ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁴ Max-Planck Institute for Astronomy (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁵ Independent Researcher, Porto, Portugal

^★★ Corresponding author: aaron.empey@ucdconnect.ie

Received: 30 July 2025
Accepted: 20 October 2025

Abstract

Context. The dipper subclass of young stellar objects (YSOs) are characterised by frequent dips in their light curves. Irregular dippers do not show periodic signatures and have dips accounting for significant proportions of their photospheric flux. Given the short timescales on which these dips occur, their driving mechanisms are linked to the inner circumstellar disc dynamics.

Aims. We present the first multi-epoch analysis of 16 irregular dippers observed with X-Shooter. Investigating the properties of their dips, and in particular the analysis of the dust characteristics, we aim to understand the root of their variability, and get a glimpse of the inner disc behaviour.

Methods. We employed a novel approach to measure the properties of the dips, by combining class III templates with Gaia multiepoch photometry to construct the intrinsic photosphere of the objects. We measured several dip properties including the depth of the dips, near-infrared (NIR) excesses, and their optical depths as a function of wavelength.

Results. We record 20 significant dips that range in their dip properties and show no relation to one another. In almost all cases, the low optical depths and small NIR excesses are observed. Comparison of their optical depths with grain opacity models show that the dips can be explained by the presence of dust substructures containing processed grains obscuring their photospheres and/or their discs. These grain distributions can have maximum sizes as large as 20 μm and in many cases have almost grey-like extinction, while some require a strong scattering component.

Conclusions. The findings highlight the extent of the irregularity of dippers, but also link it to the dust dynamics in the inner regions of the circumstellar discs. The dust substructures causing the variability require processed dust grains to be lifted above the disc into the line of sight. Possible lifting mechanisms including disc winds, unstable accretion columns, and disc warps are discussed.